Solid state polymerization of polyesters with low diffusion resistance prepolymer granules

ABSTRACT

Polyester prepolymer granules with open-ended cavities provide greatly improved solid state polymerization rates by improving reaction by-product diffusion characteristics.

This is a division of application Ser. No. 08/137,876, filed Oct. 15,1993.

BACKGROUND

This invention relates to solid state polymerization of low diffusionresistance granules of polyester prepolymers.

High molecular weight polyesters, such as poly(ethylene terephthalate)(PET) and poly(ethylene napthalate) (PEN), are usually produced by acombination of melt polymerization and solid state polymerization (SSP)processes. Polyester prepolymers with relatively low molecular weight(evidenced by a relatively low intrinsic viscosity, "I.V.") are producedin a melt polymerization process. The prepolymer melt is typicallyextruded through a die with multiple orifices to form molten strandsthat are quenched, solidified, and chopped into granules or pellets.There are, of course, other methods to convert polyester melt intogranular form. Hereafter, for convenience, polyester prepolymergranules, regardless of their shape or method of forming, will bereferred to as pellets. The prepolymer pellets are further polymerizedin solid phase to a desired high molecular weight or I.V., attemperatures below the melting point but above the glass transitiontemperature.

Generally, the shape of the orifices in the extrusion die used topelletize prepolymer is round, but orifices of other simple shapes canbe used. Drooping of the molten prepolymer strands and/or someflattening actions of the pelletizing equipment, cause the cross-sectionof the resulting prepolymer pellets to be somewhat elongated and notexactly round; i.e., approximately elliptical.

Solid state polymerization (SSP) is conducted under vacuum or in astream of purge gas such as nitrogen. Overall, SSP involves two majorsteps: (1) chemical reactions and (2) diffusion of reaction by-products.The chemical reactions are transesterification and esterification.

Transesterification generates ethylene glycol (EG) as the by-product andesterification generates water as the by-product. To force the polyesterprepolymer to further polymerize, reaction by-products must beeffectively removed as they are generated. By-products generated by thereactions diffuse from the interior to the surfaces of the pellets wherethey are removed by vacuum or an inert gas stream. Since resistance toby-product diffusion from the surfaces of the pellets to the bulk of thegas phase is negligible, only the chemical reaction rates and by-productdiffusion from the interior to the surfaces of the pellets are majorfactors in polymerization performance.

Enhanced chemical reaction rates are achieved by using the maximumallowable reaction temperature, slightly below the temperature at whichpolymer pellets start to stick together or to the reactor wall.

Resistance to diffusion of by-products can be reduced by reducingprepolymer particle size. However, at a fixed temperature, smallerparticles have higher tendencies to stick. Therefore, lower reactiontemperatures are required if particle size is reduced. Moreover,excessively small particles are hard to handle. Therefore, it is noaccident that most of the polyester pellets weigh between 0.015 to 0.03grams per pellet. Another way to reduce the by-product diffusionalresistance while maintaining the pellet size is to shape the pelletcross-section like stars, crosses, dog bones and the like. However, suchshapes are generally undesirable because the increased inter-pelletcontact area of these shapes increases the tendency to stick. Therefore,practically all standard prepolymer pellets have round, elliptical,square, rectangular or other simple cross-sections.

Heretofore, two methods have been proposed to reduce diffusionalresistance to by-products while maintaining a suitable prepolymerparticle size and without increasing polymer sticking tendency. U.S.Pat. No. 3,586,647 proposes foamed pellets which are formed byincorporating nitrogen or a foaming agent into the prepolymer meltbefore pelletizing. Foamed pellets polymerize only slightly faster thansolid pellets. In U.S. No. 4,755,587 it is proposed to use porouspellets with interconnected voids. Although porous pellets solid statepolymerize much faster than standard pellets, the formation of porouspellets requires expensive additional steps, such as grinding,compacting, granulating, and classifying. Moreover, porous pellets tendto generate large amounts of fines.

It is the purpose of this invention to improve the polymerizationprocess by providing an improved polyester prepolymer pellet form thatoffers reduced diffusion resistance to the reaction by-products and,therefore, higher polymerization rates, without the disadvantages ofexisting techniques.

SUMMARY OF THE INVENTION

This invention is, in one embodiment, a polyester granule havingimproved gas diffusion characteristics that can be solid statepolymerized into a high molecular weight polyester comprising apolyester prepolymer formed into shaped pellets of a size and weightsuitable for solid state polymerization and having one or moreopen-ended cavities.

In another embodiment the invention is an improvement in a process forsolid state polymerizing a polyester prepolymer into a high molecularweight polyester at elevated temperature, the improvement that comprisesproviding a polyester prepolymer that is formed into shaped pellets of asize and weight suitable for solid state polymerization, each having oneor more open-ended cavities.

DESCRIPTION OF DRAWINGS

FIG. 1 is a plot of polymer I.V. against reaction time for the solidstate polymerization of PET pellets with different cross sections.

FIG. 2 is a plot of polymer I.V. against reaction time for the solidstate polymerization of PEN standard pellets and pellets with open-endedcavities.

DETAILED DESCRIPTION

This invention involves the solid state polymerization of polyesterprepolymers that are shaped in a manner to provide one or moreopen-ended cavities inside each shaped particle. The shape, size andcapability to effectively reduce gas diffusion resistance results ingreatly improved solid state polymerization rates that provide a numberof advantages in both newly designed polymerization plants as well as inexisting plants. A listing of some of these advantages follows thisdetailed description.

The invention is applicable to virtually any polyester that can be solidstate polymerized. The most common polyesters suitable for use in theinvention have at least about 75 mole percent of their acid moietiesprovided by an aromatic dicarboxylic acid, such as terephthalic acid,isophthalic acid, or a naphthalenic dicarboxylic acid (preferably 2,6-)with the diol moieties provided by glycols such as ethylene glycol,butylene glycol, 1,4-dimethylol cyclohexane and the like or aromaticdiols such as hydroquinone and catechol. Such polyesters can containother dicarboxylic acids such as adipic acid, isophthalic acid, sebacicacid, and the like. Poly(ethylene terephthalate) (PET), poly(ethyleneisophthalate), poly(ethylene naphthalate), and poly(butyleneterephthalate) homopolymers are representative examples of suchpolyesters. Poly(ethylene naphthalate)s are especially suitable. Blendsof various polyesters can also be polymerized in the process.

The diacid component in the polyesters is normally comprised of alkyldicarboxylic acids which contain from 4 to 36 carbon atoms, diesters ofalkyl dicarboxylic acids which contain from 6 to 38 carbon atoms, aryldicarboxylic acids which contain from 8 to 20 carbon atoms, diesters ofaryl dicarboxylic acids which contain from 10 to 22 carbon atoms, alkylsubstituted aryl dicarboxylic acids which contain from 9 to 22 carbonatoms, or diesters of alkyl substituted aryl dicarboxylic acids whichcontain from 11 to 22 carbon atoms. The preferred alkyl dicarboxylicacids will contain from 4 to 12 carbon atoms. Some representativeexamples of such alkyl dicarboxylic acids include glutaric acid, adipicacid, pimelic acid, and the like. The preferred diesters of alkyldicarboxylic acids will contain from 6 to 12 carbon atoms. Arepresentative example of such a diester of an alkyl dicarboxylic acidis dimethyl azelate. The preferred aryl dicarboxylic acids contain from8 to 16 carbon atoms. Some representative examples of aryl dicarboxylicacids are terephthalic acid, isophthalic acid, and orthophthalic acid.The preferred diesters of aryl dicarboxylic acids contain from 10 to 18carbon atoms. Some representative examples of diesters of aryldicarboxylic acids include dimethyl terephthalate, dimethylisophthalate, dimethyl orthophthalate, dimethyl naphthalate, diethylnaphthalate and the like. The preferred alkyl substituted aryldicarboxylic acids contain from 9 to 16 carbon atoms and the preferreddiesters of alkyl substituted aryl dicarboxylic acids contain from 11 to15 carbon atoms.

The diol component of the polyester prepolymers is normally comprised ofglycols containing from 2 to 12 carbon atoms, glycol ethers containingfrom 4 to 12 carbon atoms, and polyether glycols having the structuralformula

    HO--(A--O).sub.n --H

wherein A is an alkylene group containing from 2 to 6 carbon atoms andwherein n is an integer from 2 to 400.

Branched polyester prepolymers can also be used. Such branchedpolyesters normally contain branching agents that have three or morefunctional groups; preferably three or four. Reactive functional groupscan be carboxyl or aliphatic hydroxyl groups and can optionally containboth carboxyl groups and hydroxyl groups. Examples of acidic branchingagents include trimesic acid, trimellitic acid, pyromellitic acid,butane tetracarboxylic acid, naphthalene tricarboxylic acids, andcyclohexane-1,3,5-tricarboxylic acids. Representative examples ofhydroxyl branching agents (polyols) include glycerin, trimethylolpropane, pentaerythritol, dipentaerythritol, 1,2,6-hexane triol, and1,3,5-trimethylol benzene. Generally, from 0 to 3 percent (based uponthe total diol component) of a polyol containing from 3 to 12 carbonatoms will be used.

High strength polyesters that utilize at least one bis-hydroxyalkylpyromellitic diimide in their diol component such as those described inU.S. Pat. No. 4,605,728 can also be used.

The polyester prepolymers (starting polyesters) may be made by anysuitable method but are typically prepared by conventional meltpolymerization techniques using temperatures, catalysts, and stabilizerswell known in the polyester art. These polyester prepolymers have arelatively low initial starting I.V. Prepolymers should preferably havea starting I.V. (as measured in a 60:40 by weightphenol:1,1,2,2-tetrachloroethane solvent system at a temperature of 30°C.) of from about 0.25 dl/g to about 0.75 dl/g. An I.V. of 0.25 dl/g isuseful because it has been found to be about the lower limit that PETcan be pelletized without shattering.

In the practice of this invention, the prepolymer is formed into shapedpellets (granules) with one or more open-ended cavities prior to solidstate polymerization. Each of the pellets contains one or moreopen-ended cavities but retains the same general outer cross-sectionalshape as standard solid pellets. Pellets will normally weigh from about0.1 grams to 10 grams per 100 pellets. Pellets weighing from about 1 to3 grams per 100 pellets are commonly used commercially and arepreferred. An "open-ended" cavity is a cavity or hole with at least oneend or side open to allow the carrier gas to enter the interior ofpellets. It may be open on both ends and may or may not be open to theside. Suitable pellets can be advantageously formed by extruding theprepolymer through a specially designed die, quenching the extrudedstrands and chopping the solidified strands into pellets. The cavitiesin the pellets formed in this way are usually open at both ends. Pelletswith both ends open are preferred. If one of the cavities is also opento the side, there will be an open gap on the side of the pellet. Thepellets will generally be cut into lengths of from about 1 mm to 10 mm.If too short, they tend to shatter and form fines and if too long theyhave a greater tendency to Stick. Lengths of 2 to 3 mm are preferred forpellets in the size range of about 1 to 3 grams per 100 pellets.

All else being equal, the sticking tendencies of polymer pellets duringSSP increase with increasing specific inter-pellet contact area, whichis defined as the inter-pellet contact area per unit weight of polymer.With similar general cross-sectional shape and length as standardpellets, the preferred pellets with cavities have approximately the samesticking tendency as the standard pellets. The presence of one or morecavities in each pellet greatly shortens the by-product mean diffusionpath and increases surface area thereby lowering the overall by-productdiffusional resistance within the pellet. Furthermore, the open cavitiesallow the purge gas to flow into or pass through the interior of thepellets to sweep away reaction by-products as soon as they diffuse tothe inner surfaces. Therefore, polyester prepolymer pellets withopen-ended cavities solid state polymerize much faster than foamedpolyester prepolymer pellets with equal void fraction.

The simplest pellets with open-ended cavities are those with "C"-shapedor "O"-shaped cross-sections formed by a extrusion-pelletizingtechnique. These pellets can be produced by discharging the prepolymermelt from the melt reactor through an extrusion die having "C"-shapedorifices, quenching the extruded strands, and chopping the solidifiedstrands with a pelletizer. As the extruded melt strands emerge from thedie face, each will have a "C"-shaped cross-section. This allows air toenter the interior of each forming strand. Depending on the design ofthe die orifices, the polymer properties, and the extrusion conditions,the gaps of the extruded strands may or may not close to form tubes. Ifthe gaps close, pellets with "O"-shaped cross-sections result.Otherwise, pellets with "C"-shaped cross-sections result. In general, ifthe gap in each die orifice is narrow enough, the polymer I.V. is highenough, and/or the die temperature is low enough, pellets with"O"-shaped cross-sections will be produced. For convenience, pelletswith "C"- and "O"-shaped cross-sections are referred to as C-pellets andO-pellets respectively. The cavity in a C-pellet is open to the side butthe cavity in an O-pellet is not. The general shape of thecross-sections of C-pellets and O-pellets are approximately ellipticalalthough the general orifice shape of the die used may be round.

To prevent possible interlocking of C-pellets, and thus polymer stickingtendency, the gap in each C-pellet should be narrower than half thepellet diameter, preferably narrower than one fifth the pellet diameter.

Prepolymer pellets with multiple open-ended cavities can also beproduced using properly designed extrusion dies. Since dies that producepellets with multiple cavities are more difficult to design andfabricate it is generally preferred that the number of cavities in eachpellet be limited.

For some applications it may be desirable to use extrusion dies withmandrels or similar devices. A mandrel is a hollow pin inserted in thecenter of a die orifice that forms the cavity. Air or nitrogen can beintroduced though the center of the mandrel to fill the center of theextrudate, thus reducing the possibility of the cavity closing whenquenched or in further processing.

Although it is preferable to produce prepolymer pellets with open-endedcavities by simply replacing a standard melt reactor discharging diewith a die having "C"-shaped or other desired shaped orifices, it ispossible to convert standard pellets with an additional extrusion step.Although the general cross-sectional shape of the pellets withopen-ended cavities is preferably round or elliptical, it may beapproximately square, rectangular, or of other simple geometrical shape.

For example, various shapes, such as triangles, rounded triangles andsquares can be formed by the so-called adhesive profile extrusiontechniques disclosed in the book "Polyester Fibres" Chemistry andTechnology, by Herman Ludewig, Wiley Publishing, 1964. While this bookdiscusses fiber shapes, hollow strands chopped into pellets are formedin the same manner.

For solid state polymerization it is necessary that the prepolymer bepartially crystalline to reduce sticking. Crystallinity should generallybe above about 30 percent and more typically above about 40 percent.Since extruded pellets will be substantially amorphous, it is necessaryto process the pellets to increase crystallinity to the desired level.This is generally done by heating. Crystallization is carried out in anysuitable equipment in which the polyester granules can be heated tocrystallization temperatures without sticking. Agitation normally helpsprevent sticking. Crystallization can also be carried out in a fluidizedbed crystallizer. Fluidization is accomplished by utilizing a gas flowrate sufficient to cause the pellets to be fluidized in the crystallizerwith or without mechanical vibration. Inert gas or air can be used.Since very large quantities are required for fluidization, air is mosteconomical.

Crystallization residence time is generally in the range of about 2 toabout 20 minutes and preferably from about 5 to about 10 minutesdepending on temperature. In the case of poly(ethylene terephthalate),air at temperatures in the range of about 140° C. to about 215° C. isused for heating. Air temperatures in the range of about 190° C. toabout 200° C. are preferred.

Density is a convenient measure of crystallinity and, in general,poly(ethylene terephthalate) prepolymer is suitably crystallized to adensity of at least about 1.37 g/cc. A density of at least about 1.38g/cc is preferred and 1.39 g/cc more preferred.

The polyester granules can also be crystallized in a high frequencyenergy field ranging from about 20 megahertz to about 300 megahertz asdescribed in greater detail in U.S. Pat. No. 4,254,253.

Solid state polymerization (SSP) of the pellets with open-endedcavities, low diffusion resistance, pellets is conducted at conditionssuitable for polymerization of standard solid pellets of similar size.Generally SSP is conducted at a temperature of about 10° C. to about 50°C. below the melting point of the prepolymer. For PET a temperaturerange of 200° C. to 245° C., and preferably 210° C. to 230° C. isappropriate. The polymerization is conducted under vacuum or in a streamof inert gas in a suitable reactor. Appropriate conditions are the sameas suitable for standard pellets or porous pill pellets. Suitableconditions are detailed in U.S. Pat. No. 4,977,196 at col. 8, line 45 tocol. 10, line 62 as well as in numerous other patents including U.S.Pat. Nos. 4,755,587, 4,876,326 and 4,849,497. The disclosure of U.S.Pat. No. 4,977,196 is incorporated herein by reference.

The advantages of the prepolymer pellets with open-ended cavities andother aspects of this invention are demonstrated in the followingIllustrative Embodiments.

Illustrative Embodiments EXAMPLE 1

One standard strand die (Die S) and two "C"-profile dies (Dies A and B)were used to prepare standard and pellets with open-ended cavities ofPET prepolymer having an I.V. of 0.59 dl/g and a carboxyl content of 27meg/kg for solid state polymerization tests. The standard strand die hada 3/16 inch diameter orifice. Each of the two "C"-profile dies has a"C"-shaped orifice. The dimensions of the orifice of each of the two"C"-profile dies is defined by two concentric circles and a gap. Theinner circle and the gap make up the "land" area within the orifice. Theorifice dimensions of the three dies are as follows:

    ______________________________________                                        Die     OD, in         ID, in  Gap, in                                        ______________________________________                                        S        3/16          --      --                                             A       1/4            1/16    1/48                                           B       1/4            3/32    1/32                                           ______________________________________                                    

It is not necessary that the two circles that define the orifice of a"C"-profile die be concentric. Both Die A and B are examples of thesimplest designs. The three dies were used in conjunction with a 11/4inch laboratory extruder and a pelletizer to prepare pellets withopen-ended cavities and standard pellets. Four different types of PETprepolymer pellets, each with different cross-sections, prepared usingthe three dies were chosen for the solid state polymerization (SSP)tests. Pellet preparation resulted in practically no change in I.V. orcarboxyl content.

Table I list the characteristics of the pellets and the extrusionconditions used to prepare them. The four types of pellets aredesignated S-Pellets, O-pellets, Q-Pellets, and C-pellets respectively.S-Pellets were standard solid pellets prepared with Die S; they wereused as the control. O-Pellets were pellets with "O"-shapedcross-sections prepared with die A. Q-Pellets and C-Pellets were pelletswith "C"-shaped cross-sections prepared with Die B. The main differencebetween Q-Pellets and C-Pellets was in the dimension of the gap;Q-Pellets had very narrow gaps and "C"-Pellets had wider gaps.

Although the standard strand die used to prepare the S-Pellets has around orifice, the cross-sections of the S-pellets were elongatedbecause the melt strands drooped before quenching. Each of the threetypes of pellets with open-ended cavities were prepared using a"C"-profile die with the land gap of the die orifice oriented upward. Asthe melt strands cooled, the gaps in the strand tended to close orshrink back. These actions had the effect of offsetting the elongationof the cross-section caused by drooping. Therefore, all three types weremore nearly round in cross-section than standard pellets. This isevidenced by their lower axis ratios. This is beneficial because theless elongated the cross-sections, the lower the pellet stickingtendency during solid state polymerization. All four types of pelletshad nearly equal average pellet weight (between 1.82 and 1.84 grams per100 pellets). Since the standard pellets have more elongated orelliptical cross-sections, they will have a somewhat higher stickingtendency than the three types of pellets with open-ended cavities. Ifthe cross-sections were the same, the sticking tendency would be thesame. The four types of PET pellets were solid state polymerized in asmall scale static bed reactor at the same SSP temperature todemonstrate the SSP rate advantages of the three types of pellets withopen-ended cavities over standard pellets.

The static-bed reactor used was made from a 24-inch long glass tube with1-inch ID. The bottom of the reactor was connected to a small purge-gassupply tube which coiled up around the reactor. The only purge gas usedwas nitrogen. The reactor with its purge-gas supply tube was immersed ina thermostated oil bath that heated the reactor and the incomingnitrogen. For each SSP run, 80 grams of PET pellets were used. A streamof nitrogen was passed through the reactor at a rate of 18 standardcubic feet per hour at all times during the run. Because of the longlength of the purge-gas supply tube immersed in the oil bath, thenitrogen was heated to the oil temperature as it reached the bottom ofthe reactor.

In each run, the pellets were first crystallized and dried at 180° C.for 60 minutes. Then reactor temperature was raised to 220° C. over 10minutes. The SSP temperature of 220° C. was used because it is near themaximum reactor temperature allowable in the continuous commercial-scaleSSP operations in the production of high I.V. PET. The SSP step lasted23 hours. Samples were taken at various intervals for I.V. tests. TableII lists the I.V.'s of the samples taken during the runs.

The I.V. data listed in Table II were further plotted against the SSPtime in FIG. 1 to compare the SSP rates of the four types of pellets. Itcan be seen from FIG. 1 that all the three types of pellets withopen-ended cavities solid state polymerize much faster than the standardpellets.

C-Pellets, with largest cavities and open gaps, solid state polymerizefastest: O-Pellets, with the smallest cavities and with no open gaps,polymerize more slowly, although not by much.

From FIG. 1, the reaction times required for the four types of pelletsto achieve two product I.V.'s, 0.84 and 0.95 dl/g, were determined.These I.V.'s are required for bottle and food tray applicationsrespectively. From these SSP time requirements and the bulk densitydata, the average SSP rates, the SSP rate ratios, and the productivityadvantages over the standard pellets were calculated for the productionof 0.84 and 0.95 I.V. products. The result are listed in Table III.

It can be seen from Table III that, between I.V.'s of 0.59 and 0.84dl/g, O-Pellets, Q-Pellets, and C-Pellets solid state polymerize fasterthan the standard pellets by 34.4, 41.3, and 46.2 percent respectively.Since productivity of a continuous SSP process is based on polymerweight per hour, productivity advantages of the pellets with open-endedcavities over the standard pellets can be calculated by taking intoaccount the bulk densities of the four types of pellets. They are: 29.3,31.9, and 30.2 percent respectively for O-Pellets, Q-pellets and Cpellets. It can be seen that the best productivity improvement isrealized by replacing the standard pellets with Q-Pellets, which have"C"-shape cross-sections with very narrow open gaps. Thus, Q-pelletsappear to have the best combination of SSP rate and bulk density amongthe three types of pellets with open-ended cavities.

EXAMPLE 2

The standard die and Die B used in Example 1 were used to preparestandard solid pellets and pellets with open-ended cavities ofpoly(ethylene naphthalate) (PEN) for SSP tests. The PEN prepolymer usedhad an I.V. of 0.48 dl/g as measured in a 60:40 phenol:tetrachloroethanesolvent system at a temperature of 30° C. and a carboxyl content of 12meg/kg. Pellet preparation had little or no effect on I.V. or carboxylcontent. The standard pellets obtained had a pellet weight of 1.98 gramsper 100 pellets, a major axis of 2.58 mm, a minor axis of 2.00 mm, and abulk density of 0.78 g/cc. The pellets with open-ended cavities preparedwith Die B had a pellet weight of 1.97 grams per 100 pellets, an"O"-shaped cross-section with 2.50 mm major axis and 2.38 mm minor axis,an approximate cavity diameter of 0.71 mm, and a bulk density of 0.71g/cc. Pellet length was 3 mm for both types of pellets.

These two types of PEN prepolymer pellets were solid state polymerizedin the same small-scale static-bed reactor used in Example 1. Except foran additional devolatilization step, the SSP experimental proceduresused in this example were similar to those used in Example 1. For eachrun, the PEN pellets were first devolatilized at 120° C. for 4 hours.Then the PEN pellets were crystallized and dried at 200° C. for 1 hour.Then the bed temperature was raised to 250° C. to effect SSP whichlasted 24 hours. At all times throughout the run, the polymer bed waspurged with a stream of nitrogen. Table IV lists the I.V.'s of thesamples taken during the SSP runs with the PEN standard pellets andpellets with open-ended cavities. The I.V. data in Table IV were alsoplotted in FIG. 2 to compare the SSP rates of these two types of PENpellets.

Two major applications of high I.V PEN are packaging and industrialfiber. Packaging applications require an I.V. of about 0.65 dl/g, andindustrial applications, about 0.75 dl/g. From FIG. 2, the standardpellets (S-pellets) required 8.3 and 18.0 hours, whereas the pelletswith open-ended cavities (O-Pellets) required 4.9 and 10.1 hoursrespectively to achieve these I.V.'s. Thus the pellets with open-endedcavities solid state polymerized faster than the standard pellets by 69percent between 0.48 and 0.65 dl/g, and by 78 percent between 0.48 and0.75 dl/g. Taking into account the lower bulk density of the pelletswith open-ended cavities, productivity gains of 54 and 62 percentrespectively can be achieved by replacing the standard pellets with thepellets with open-ended cavities in the production of PEN resins forpackaging and industrial fiber applications.

Compared with Example 1, it can he seen that even greater productivitygains can be realized with PEN than with PET by the use of theprepolymer pellets of this invention. This is because PEN has a lowervapor permeability or higher vapor barrier properties than PET.

Some Advantages of this Invention

Prepolymer pellets with open-ended cavities offer the followingadvantages over standard prepolymer pellets:

Examples 1 and 2 demonstrate that pellets with open-ended cavities solidstate polymerize much faster than standard pellets. Therefore, with anexisting SSP plant, greater productivity can be achieved by usingpellets with open-ended cavities. In a new plant a smaller SSP reactoris required for equal productivity. In both cases, the production costsare lower.

Even if I.V. is uniform throughout each prepolymer pellet before SSP, itwill vary within the pellet after SSP. This is because of diffusionaldifferences of by-products from within the pellet. As a result, the I.V.will be highest at the surface and lowest at the center of a solidpellet. Since the maximum by-product diffusion path length can be morethan halved by introducing a cavity in the pellet, such pellets have amuch narrower I.V. distribution than standard pellets. Narrower I.V.distribution offers better polymer properties and processability. Thisis especially important in extrusion-blow molding.

Because of the higher SSP rates of pellets with open-ended cavities, thecapability of an existing SSP process can be enhanced. For example, in acontinuous SSP reactor with a residence time of 10 hours, the highestachievable product I.V. is, as shown in FIG. 1, about 0.88 dl/g withstandard PET prepolymer pellets; sufficient for bottle application butinsufficient for food tray application. If Q-Pellets or C-Pellets areused, products with I.V.'s exceeding 0.95 dl/g are obtained; sufficientfor bottle and food tray applications. The use of pellets withopen-ended cavities enables an existing SSP process to produce highermolecular weight products for more demanding applications.

Use of pellets with open-ended cavities can also enable loweringprepolymer I.V. while maintaining the productivity and high product I.V.on an existing SSP line. Lower prepolymer I.V. leads to greaterproductivity for the melt process used to produce the prepolymer.Obviously, with pellets with open-ended cavities, not only highermolecular weight products can be produced with an existing SSP process,but the feed polymer I.V. can also be lowered.

Low acetaldehyde contents are desirable for bottle resins. By usingpellets with open-ended cavities with lower I.V., products with loweracetaldehyde content can be obtained without sacrificing productivity.

                  TABLE I                                                         ______________________________________                                        Pellet Characteristics                                                        and Extrusion Conditions used to Prepare Them                                 Pellet                                                                        Shape      S        O         Q      C                                        ______________________________________                                        Die Used   Standard A         B      B                                        Barrel temp, °F.                                                                  540      540       540    540                                      Die temp, °F.                                                                     540      490       490    540                                      Screw RPM  33       33        33     50                                       Pelletizer 10       10        10     15                                       Speed Setting                                                                 Pellet wt, gm                                                                            0.0182   0.0184    0.0184 0.0182                                   Bulk density                                                                             0.793    0.763     0.740  0.706                                    g/cc                                                                          Pellet length,                                                                           3        3         3      3                                        mm                                                                            Major axis, mm                                                                           2.72     2.45      2.48   2.58                                     Minor axis, mm                                                                           1.87     2.24      2.35   2.36                                     Axis ratio 1.45     1.09      1.06   1.09                                     Pellet cavity                                                                            --       0.25      0.35   0.72                                     diameter, mm                                                                  Gap in pellet,                                                                           --       --        0.11   0.39                                     mm                                                                            ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        I.V.'s of Samples of PET Pellets Taken During Solid State                     Polymerization a 220° C.                                               SSP        S       O          Q     C                                         Time,hr    Pellets Pellets    Pellets                                                                             Pellets                                   ______________________________________                                        0          0.590   0.589      0.590 0.591                                     1          0.615   0.641      0.640 0.660                                     2          0.661   0.689      0.695 0.700                                     3.5        0.714   0.755      0.758 0.765                                     5          0.765   0.810      0.812 0.819                                     7          0.805   0.864      0.875 0.882                                     10         0.883   0.939      0.956 0.958                                     14         0.955   1.025      1.050 1.067                                     18         1.013   0.126      1.140 1.152                                     23         1.105   1.192      1.216 1.221                                     ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Reaction Rate and Productivity improvements of Pellets with                   Open-ended Cavities over Standard Pellets in Continuous                       Solid State Polymerization                                                    Pellet Type  S       O         Q     C                                        ______________________________________                                        A. Product I.V. = 0.84 dl/g for bottle applications                           SSP time     8.2     6.1       5.8   5.6                                      Required, hr                                                                  Avg, SSP rate                                                                              0.0305  0.0410    0.0431                                                                              0.0446                                   dl/g/hr                                                                       SSP rate ratio                                                                             1.000   1.344     1.413 1.461                                    % productivity                                                                             --      29.3      31.9  30.2                                     Advantage                                                                     B. Product I.V. = 0.95 dl/g for food tray applications.                       SSP time     13.8    10.2      9.7   9.4                                      required, hrs                                                                 Avg. SSP rate                                                                              0.0261  0.0353    0.0371                                                                              0.0333                                   dl/g/hr                                                                       SSP rate ratio                                                                             1.000   1.352     1.421 1.467                                    % Productivity                                                                             --      30.1      32.6  30.6                                     advantage                                                                     ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        I.V.'s of Samples Taken During Solid State Polymerization                     of PEN Standard Pellets and Pellets with Open-ended Cavities                                          Pellets with                                                        Standard  Open-ended                                            SSP time,     Pellets   Cavities                                              hr            (S-Pellets)                                                                             (O-Pellets)                                           ______________________________________                                        0             0.4830     0.4830                                               1            0.506      0.526                                                 2            0.534      0.562                                                 3.3          0.570      0.613                                                 5            0.595      0.653                                                 7            0.631      0.696                                                 10           0.670      0.755                                                 14           0.711      0.792                                                 18           0.754      0.845                                                 24           0.795      0.887                                                 ______________________________________                                    

What is claimed is:
 1. A polyester prepolymer in the form of pellets,wherein the pellets are prepared from a polyester prepolymer that isextruded in tubular form, have a hollow portion extending throughout thelength of the pellet, and are open-ended, and wherein the prepolymer hasan intrinsic viscosity between 0.25 dl/g and 0.75 dl/g.
 2. The pelletsof claim 1 having a simple general cross-sectional shape, a weight ofabout 0.1 grams to 10 grams per 100 pellets and a length of about 1 mmto 5 mm.
 3. The pellets of claim 1 in which the weight is between 1 gramand 3 grams per 100 pellets and the length is 2 mm to 3 mm.
 4. Thepellets of claim 1 in which the polyester prepolymer has an intrinsicviscosity of at least 0.25 dl/gram as measured in a 60:40 phenoltetrachloroethane solvent system at a temperature of 30° C. prepolymerhas an intrinsic viscosity between 0.25 dl/g and 0.75 dl/g.
 5. Thepolyester of claim 1 wherein the pellets are open at both ends and havean "O"-shaped cross-section.
 6. The polyester of claim 1 wherein thepellets are open at both ends and open to the side, and the pellets havea "C"-shaped cross-section.
 7. The polyester of claim 6 wherein the sideopening of the "C"-shaped pellets is narrower than one-fifth the pelletdiameter.
 8. The pellets of claim 7 in which there is one cavity open tothe side forming an open gap of between 0.01 mm and 1.0 mm.
 9. Thepolyester of claim 1 wherein the polyester is a poly(ethyleneterephthalate) polymer.
 10. The polyester of claim 1 wherein thepolyester is a poly(ethylene naphthalate) polymer.
 11. A solid-statepolymerized polyester product in the form of shaped pellets, wherein thepellets are prepared from a polyester prepolymer that is extruded intubular form and shaped into open-ended pellets prior to solid statepolymerization, and wherein the polyester prepolymer has an intrinsicviscosity between 0.25 dl/g and 0.75 dl/g.
 12. The polyester of claim 11wherein the pellets are open at both ends and have an "O"-shapedcross-section.
 13. The polyester of claim 11 wherein the pellets areopen at both ends and open to the side, and the pellets have a"C"-shaped cross-section.
 14. The polyester of claim 13 wherein the sideopening of the "C"-shaped pellets is narrower than one-fifth the pelletdiameter.
 15. The polyester of claim 11 wherein the polyester producthas an IV of 0.65 dl/g or more.
 16. The polyester of claim 11 whereinthe polyester product has an IV of 0.75 dl/g or more.
 17. The polyesterof claim 11 wherein the polyester product has a reduced acetaldehydecontent.
 18. The pellets of claim 10 having a simple generalcross-sectional shape, a weight of about 0.1 grams to 10 grams per 100pellets and a length of about 1 mm to 5 mm.
 19. The pellets of claim 10in which the weight is between 1 gram and 3 grams per 100 pellets andthe length is 2 mm to 3 mm.
 20. The pellets of claim 14 in which thereis one cavity open to the side forming an open gap of between 0.01 mmand 1.0 mm.